It is often desirable to enhance certain surface characteristics of substrates such as, for example, exterior and interior body parts of automobiles. In particular, with regard to exterior automobile body parts, base coat/clear coat painted finishes have become quite popular. These finishes are conventionally applied by first painting a base coat, which may include metallic pigments, onto the body part and then painting a clear coat over the base coat. Both the base coat and the clear coat are typically applied by spraying. The base coat/clear coat painted finish thus can provide a "wet look" finish, with or without a metallic appearance, and can be used to produce a very high quality finish. In addition, because the clear coat protects the base coat, greater latitude can be used in pigment selection and loading in the base coat without the weatherability of the finish being unacceptably decreased.
Base coat/clear coat finishes, however, and particularly conventionally painted metallic finishes, pose significant pollution control problems arising from the evaporation of volatile paint solvents. Moreover, they are difficult to provide on plastic automobile body panels which are becoming a more popular mode of construction in the industry.
Accordingly, decorative composite surfacing films have been developed which can be applied to a substrate, such as an automobile body part, to impart the appearance of base coat/clear coat painted finishes. Exemplary films and methods for making and applying the same to automobile body parts are disclosed in U.S. Pat. Nos. 4,810,540; 4,931,324; 4,943,680; and 5,342,666 which are all incorporated by reference herein. These patents disclose composite surfacing films having a clear coat layer and an underlying base coat layer adhered thereto. After the films have been formed, they may be bonded to a plastic film or sheet and thermoformed into the final part; thermoformed into an insert for placement in a mold for resin injection to make the part; or placed directly into a mold, without preforming, for resin injection to make the part. The composite surfacing films thus provide a high quality decorative finish simulating that of a base coat/clear coat painted finish and substantially reduce or eliminate the emission control problems previously faced by manufacturers who wished to produce automobiles with base coat/clear coat finishes.
Another method of producing film materials of this type is disclosed in PCT International Application No. WO 94/03337 to Duhme et al. Duhme et al. discloses forming a clear coat layer by extrusion and then laminating the extruded clear coat layer to a PET carrier. A color layer may be cast onto a different carrier and then, after drying and removal of the carrier, laminated to the clear coat layer.
Extruded polymers typically take on a molecular orientation as a result of the extrusion process. During extrusion, the molecular chains become generally oriented in the machine direction as a result of the physical forces acting on the polymer as it is forced through the extrusion die. Molecular orientation, however, can have a deleterious effect on the resultant properties of the flexible surfacing film because various properties of the clear coat layer will become anisotropic. For example, the tensile strength of the clear coat layer will typically be greater in the machine direction (i.e., the direction of molecular orientation) than in the cross machine direction. The same may also be true of physical shrinkage when the surfacing film is later subjected to prolonged high temperatures, as in the thermoforming process. Dimensional instability is particularly troublesome with surfacing films as it may cause visual distortion or, in severe cases, delamination of the film or even debonding of the film from the substrate.